A cinder is a pyroclastic

Pyroclastic rock

Pyroclastic rocks or pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic...

 material. Cinders are extrusive igneous rock

Igneous rock

Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...

s. Cinders are similar to pumice

Pumice

Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...

, which has so many cavities and is such low-density that it can float on water. The colour of cinder is brown.
The following geologic

Geology

Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 characteristics define a cinder:

• Uncemented
• Vitric
• Having bubble-like cavities, called vesicles
• Measuring not less than 2.0 millimeters in at least one dimension
• Apparent specific gravity
  Specific gravity
  Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
  
   between 1.0 and 2.0
• Typical cinders are red or black in color.
• Contain numerous gas bubbles "frozen" into place as magma exploded into the air and then cooled quickly.

Cinders have been used on track surfaces and roads to provide additional traction in winter conditions.